The present invention relates generally to wireless communications. More particularly, the invention is directed to improved power monitoring in a code division multiple access (CDMA) wireless communication system.
The relative power used in each data signal transmitted by the base station in spread spectrum communications systems require control in response to information transmitted by each remote unit. The primary reason for providing such control is to accommodate the fact that in certain locations the forward channel link may be unusually disadvantaged. Unless the power being transmitted to the disadvantaged remote unit is increased, the signal quality may become unacceptable. Thus, output power must be controlled to guarantee enough signal strength received at the base station and to maintain good quality audio while minimizing the potential for interference.
Additionally, since a CDMA wideband channel is reused in every cell, self interference caused by other users of the same call and interference caused by users in other cells represents a limiting factor to the capacity of the system. Moreover, the interference coming from the neighboring base stations may not fade with the signal from the active base station as would be the case for interference coming from the active base station. Due to fading and other channel impairments, maximum capacity is achieved when the signal-to-noise ratio (SNR) for every user is, on the average, at the minimum point needed to support xe2x80x9cacceptablexe2x80x9d channel performance. A remote unit in these situations may require additional signal power from the active base station to achieve adequate performance.
Communication systems are known to employ power control methods which control transmission energy of remote units. Power control in a spread spectrum system serves two main functions. Firstly, because each remote unit""s signal in a spread spectrum system is typically transmitted in the same frequency, a majority of the noise (i.e., inversely proportional to bit energy per noise density, i.e., Eb/N0, defined as the ratio of energy per information-bit to noise-spectral density), associated with a received signal can be attributed to other remote units"" transmissions. The magnitude of noise is directly related to the received signal power of each of the other remote units"" transmissions. Thus, it is beneficial for a remote unit to transmit at a low power level. Secondly, it is desirable to dynamically adjust the power of all remote units in such a way that transmissions are received by the base station with approximately the same power level.
Dynamic power control of the mobile station""s transmitter includes two elements: open loop estimation of transmit power by the mobile station, and closed loop correction of the errors in this estimate by the base station. In open loop power control, each mobile station estimates the total received power on the assigned CDMA frequency channel. Based on this measurement and a correction supplied by the base station, the mobile station""s transmitted power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level. Differences in the forward and reverse channels, such as opposite fading may occur due to the frequency difference and mismatches in the mobile station""s receive and transmit characteristics. This may not be readily estimated by the mobile station. Each mobile station corrects its transmit power with closed loop power control information supplied by the base station via low rate data inserted into each forward traffic channel. The base station increases and reduces its outbound power between the base station and the mobile communication units in a dispatch situation, with the forward power control scheme for the base station responding to requests required by the communication units. The base station derives the correction information by monitoring the CDMA channel quality of each mobile station, compares this measurement to a threshold, and requests either an increase or decrease depending on the result.